350-501 Practice Test 12 — 15 Questions

Question 1

A service provider is migrating from LDP-based MPLS to Segment Routing (SR-MPLS) with IS-IS. After enabling SR on all routers, the adjacency segment IDs (ADJ-SIDs) are not being advertised. Which configuration is missing?

Accepted Answer

The `segment-routing mpls` command is not enabled under the IS-IS process. In IS-IS, adjacency segment IDs are programmed only when a specific prefix-SID (e.g., the loopback) is configured and SR is enabled globally. Without an SR-capable IGP process on the interface, ADJ-SIDs are not allocated. The most common missing piece is the `segment-routing mpls` command under the IS-IS process.

Answer

The router is running OSPF instead of IS-IS

Answer

The interface has the `isis adjacency-sid` command incorrectly configured

Answer

The loopback interface does not have an ip address configured

Question 2

An engineer is troubleshooting MPLS LSP connectivity. The ingress PE router has the label binding for the FEC 10.1.1.0/24, but no LSP is established. Which command should be checked on the P routers to verify the LSP path?

Accepted Answer

show mpls forwarding-table. Option B is correct because 'show mpls forwarding-table' displays the label forwarding entries and can identify missing or incorrect label assignments along the path. Option A is not specific to MPLS forwarding; option C shows LDP neighbors only; option D shows MPLS interfaces but not forwarding details.

Answer

show mpls interface

Answer

show mpls ldp neighbor

Answer

show ip route 10.1.1.0

Question 3

An MPLS network is experiencing micro-loops during convergence after a link failure. The network uses LDP and IS-IS as IGP. Which of the following solutions can prevent micro-loops during IGP convergence without requiring additional protocols?

Accepted Answer

Configure LFA (Loop-Free Alternate) on the IGP.. Loop-free alternates (LFA) provide fast convergence and micro-loop avoidance. Option B is incorrect because BGP PIC is for BGP convergence. Option C is incorrect because RSVP-TE is separate. Option D is incorrect because MPLS-TE FRR is for RSVP-TE tunnels.

Answer

Implement MPLS-TE FRR using bypass tunnels.

Answer

Enable BGP PIC (Prefix Independent Convergence).

Answer

Deploy RSVP-TE with fast-reroute.

Question 4

A network architect is designing an MPLS network that must provide fast convergence in case of a link failure. The network uses Segment Routing. Which mechanism provides sub-50ms protection by using a precomputed backup path?

Accepted Answer

TI-LFA (Topology-Independent Loop-Free Alternate). TI-LFA (Topology-Independent Loop-Free Alternate) with Segment Routing provides fast reroute by computing a backup path using segment lists that avoid the failed link. LFA provides protection but not always topology-independent. LDP FRR is older. BFD is a detection mechanism, not protection.

Answer

BFD (Bidirectional Forwarding Detection)

Answer

LFA (Loop-Free Alternate)

Answer

LDP FRR

Question 5

Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?

Accepted Answer

Labels are advertised to all neighbors without a request.. Downstream label mode means the router advertises labels to its neighbors without being asked. Option A is incorrect because downstream-on-demand sends labels only when requested. Option C is incorrect because unsolicited downstream is the same as downstream. Option D is incorrect because DoD is not shown.

Answer

Labels are sent only to the peer that advertises the prefix.

Answer

Labels are only distributed when explicitly requested.

Answer

The neighbor must request labels via a Label Request message.

Question 6

Which TWO are characteristics of Segment Routing (SR-MPLS)? (Choose two.)

Accepted Answer

It relies on source routing. Segment Routing uses source routing where the source specifies the path as a list of segments. It does not require LDP or RSVP-TE. It can use both MPLS and IPv6 data planes. Option C is wrong because SR does not require a centralized controller by default (though it can be used with one). Option D is wrong because SR allows intermediate nodes to forward based on the top segment, not IP address.

Answer

It requires a centralized controller for traffic engineering

Answer

It forwards packets based on destination IP address at each hop

Answer

It requires MPLS LDP for label distribution

Question 7

Refer to the exhibit. A network engineer is configuring a segment routing traffic engineering policy. The output shows two candidate paths. Why is path1 selected as the active path even though path2 has a bandwidth constraint?

Accepted Answer

path2 has a lower preference value.. The active path is determined by the highest preference. Path1 has preference 200, which is higher than path2's 100. Option A is incorrect because path1 is explicit, not dynamic. Option B is incorrect because both paths can be up. Option C is incorrect because color is same for both.

Answer

path1 has a shorter segment list.

Answer

path2 is down due to insufficient bandwidth.

Answer

path1 is explicit and always preferred over dynamic paths.

Question 8

Refer to the exhibit. The output shows an MPLS forwarding entry with FRR protection. What is the purpose of the backup path shown?

Accepted Answer

To forward traffic if the primary outgoing interface fails.. The backup path is used when the primary next-hop fails, providing fast reroute. Option A is incorrect because VPN label is still present. Option C is incorrect because FRR does not create a load-balancing group. Option D is incorrect because backup path is not for PHP; PHP is for penultimate hop.

Answer

To handle penultimate hop popping for the primary path.

Answer

To provide a load-balancing alternative for the primary path.

Answer

To carry the VPN label separately.

Question 9

Which TWO commands are most effective to verify that an SR-TE policy is active and forwarding traffic? (Choose two.)

Accepted Answer

show segment-routing traffic-eng policy. To verify SR-TE policy, `show segment-routing traffic-eng policy` (C) shows policy status, candidate paths, and segment lists. `show mpls forwarding-table labels` (E) shows the forwarding entry for the SR-TE label, confirming installation. Option A shows BGP-LU labels, not SR-TE. Option B shows IGP segments, not SR-TE policy forwarding. Option D shows segment list details but not active forwarding state.

Answer

show isis database verbose

Answer

show segment-routing traffic-eng segment-list

Answer

show bgp labels

Question 10

A service provider is running OSPF in their core network. After a link failure, routing convergence is taking longer than expected. The engineer suspects the issue is related to the SPF calculation. Which OSPF feature can reduce the SPF calculation time?

Accepted Answer

Incremental SPF (iSPF). Option C is correct: incremental SPF (iSPF) recalculates only affected routes, reducing convergence time. Option A (LSA pacing) is for flooding; Option B (LSDB overload) is unrelated; Option D (BFD) speeds up failure detection, not SPF calculation.

Answer

OSPF database overload protection

Answer

Bidirectional Forwarding Detection (BFD)

Answer

LSA pacing timers (pacing-interval)

Question 11

In a Layer 3 MPLS VPN, a CE router is dual-homed to two different PE routers. The PE routers advertise the same prefix to the route reflector with different route targets. What ensures that only the best path is installed in the VRF?

Accepted Answer

Each PE router independently selects the best path based on BGP attributes.. BGP path selection applies within the VRF. The route with higher local preference or shorter IGP metric to the next-hop determines the best path. Route targets are used for import/export only.

Answer

The route reflector discards duplicate prefixes.

Answer

MPLS labels ensure that only one path is used.

Answer

The CE router must advertise the same route to only one PE.

Question 12

A service provider is designing a new MPLS core network and wants to simplify traffic engineering without deploying a centralized controller. Which technology should be used to enable source-routed paths with explicit path control?

Accepted Answer

Segment Routing (SR-MPLS). Segment Routing (SR-MPLS) allows source routing by encoding paths as a list of segment IDs, enabling traffic engineering without a centralized controller. LDP only distributes labels for shortest-path forwarding. BGP-LU is for inter-domain label distribution. RSVP-TE requires a centralized controller.

Answer

BGP-LU

Answer

LDP

Answer

RSVP-TE

Question 13

An engineer is troubleshooting a BGP route reflector setup. Clients are not receiving all routes. The 'show bgp neighbors' output shows a state of 'Active'. What is the most likely cause?

Accepted Answer

Incorrect BGP neighbor statement on the route reflector or client. Option B is correct because the BGP session is in Active state, meaning the router is trying to connect but not succeeding, often due to a missing or incorrect neighbor configuration. Option A (Route reflectors do not peer with clients) is false; Option C (AS_PATH loop detection) would not prevent session establishment; Option D (next-hop reachability) does not affect BGP session state.

Answer

The route reflector does not have a full mesh with clients

Answer

The route reflector is detecting an AS_PATH loop

Answer

Next-hop reachability issue

Question 14

A service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. What is the most likely cause?

Accepted Answer

The VC ID mismatch between the two PEs. If the attachment circuit (AC) is up but no MAC learning occurs, the issue is likely that the MPLS core LSP is not available for the pseudowire. The pseudowire status may be down due to LSP issues. VC ID mismatch would cause the pseudowire to not come up. MTU or encapsulation issues would show errors.

Answer

The MTU on the core links is less than 1500

Answer

The control word is enabled on one PE but not the other

Answer

The encapsulation type on the AC is different on the two PEs

Question 15

Which MPLS VPN technology allows a service provider to offer overlapping IP addresses to different customers while using a single routing table per VPN?

Accepted Answer

MPLS Layer 3 VPN (BGP/MPLS IP VPN). Option B is correct: MPLS Layer 3 VPN (BGP/MPLS IP VPN) uses VRFs to provide separate routing tables per VPN, allowing overlapping addresses. Option A (VPLS) is layer 2; Option C (AToM) is pseudowire; Option D (MPLS TE) is traffic engineering.

Answer

Any Transport over MPLS (AToM)

Answer

VPLS

Answer

MPLS Traffic Engineering (TE)